This invention relates to solid state microwave frequency oscillators, and in particular to a push-push oscillator having a single resonator.
Frequency doubling oscillators are primarily used for signal generation at microwave frequencies above the highest operating frequency of available transistors. It is known that one way to construct a frequency doubling oscillator is by using the push-push concept. Such a push-push oscillator consists of two separate oscillator circuits, each operating at a frequency f.sub.o /2 . The output signal for each oscillator is fed to a common node for summation. If the oscillator outputs are maintained at a relative phase difference of 180.degree. C., and also have a second harmonic frequency component at f.sub.o, a signal at frequency f.sub.o with a substantially reduced frequency component at f.sub.o /2 will be present at the common node. This occurs because the two fundamental frequency components at f.sub.o /2 are 180.degree. out of phase, but the two second harmonic components at f.sub.o are in phase. The oscillator output signals' relative phase is maintained at 180.degree. by a suitably designed output load circuit which prevents in phase oscillation at f.sub.o /2 and provides the necessary conditions for out of phase oscillation at f.sub.o /2 .
The free running frequencies of the two oscillators are not identical because of tolerances in their respective circuit component values. However, a phenomenon known as injection locking takes place and can be used to insure that the frequencies of the two oscillators are locked to each other. The injection locking phenomenon is described in an article by Adler, R., entitled "A Study of Locking Phenomena in Oscillators", in Proceedings of the IEEE, Volume 61, Number 10, October 1973, pp 1380-1385. According to this article, the maximum frequency range over which injection lock can occur is inversely proportional to the external Q of the oscillators. Therefore, in the case of oscillators having low values of external Q, injection locking occurs even with a large discrepancy in their free running frequencies. Push-push oscillators are therefore characterized by low external Q values, in the range of 10-100.
Another treatment of injection locking is presented in an article by Kurokawa, K., entitled "Injection Locking of Microwave Solid-State Oscillators", in Proceedings of the IEE, Volume 61, Number 10, October 1973, pp 1386-1410.
It is also known that the frequency modulated (FM) noise of any free running oscillator is inversely dependent upon the external Q of its circuit. Consequently, the use of known push-push oscillators in applications requiring low FM noise has heretofore been precluded by the low external Q required in such oscillators for injection locking. That is to say, if injection locking is incorporated in a push-push oscillator of a known type and, at the same time FM noise is to be reduced to a minimum, impractically tight limitations on component tolerances are required.